1. Technical Field
The present invention relates to a biaxial stretch blow molding method and molding apparatus that can mold wide-mouthed containers, which are used as containers for ice cream or milk beverages, consecutively and at high speed.
2. Related Art
The plastic containers used for ice cream and milk beverages are usually formed in the shape of a truncated cone or a truncated pyramid with a wide mouth, and have a thin opening flange formed around the edge of the opening. Conventionally, such wide-mouthed containers are formed by a compression molding method or a vacuum molding method from polyethylene, polystyrene, or a polyolefine-type plastic sheet such as HIPS or the like.
In more detail, in a conventional method, sheet plastic that is taken from a plastic sheet roll is conveyed via a heating station and is heated to a suitable temperature for molding. Next, the heated plastic is conveyed to a molding station where a mobile mold presses the plastic from above with a predetermined pressing force so that the plastic is pressed downwards into a shaping mold that is positioned below the plastic and a predetermined air pressure is applied. Since a plurality of molding cavities are formed in a matrix in the shaping mold, the sheet plastic is stretched by the air pressure and pressed into each molding cavity so that the plastic is molded into the form of containers. In the next process, trimming is performed by the mobile mold that presses against the sheet plastic from above, so as to cut out each molded container. The remaining frame part of the sheet plastic, which remains after the parts corresponding to the molded containers in the matrix have been cut out, is recycled, though depending on the material used, this frame part may been discarded.
In this way, after sheet plastic of a predetermined length has been sent to the molding station, the conveying of the plastic stops until the molding is complete, and once the molding of a predetermined number of containers is complete, a predetermined amount of the sheet plastic is conveyed once more. This is to say, containers are molded in batches.
The above conventional method for molding wide-mouthed containers has the following problems. First, since wide-mouthed containers with a three-dimensional form are formed from sheet plastic, the deeper the containers are, the thinner the material thickness of the container sides becomes, which makes the containers less rigid and makes it difficult to manufacture wide-mouthed containers with sufficient drop impact strength. In particular, when a wide-mouthed container is made with a predetermined depth or more from a PET material, the strength of the container sides cannot be maintained, which makes manufacturing even more difficult.
A second problem is that around 50% of the sheet plastic is left after molding. When a material, such as PET, that is difficult to recycle is used, there is the problem of a high loss of material.
A third problem is that since the sheet plastic is conveyed to the molding station and a predetermined number of wide-mouthed containers are formed in a batch, it is not possible for a series of molding operations to be efficiently performed in a continuous manner.